Interacting Strings in Matrix String Theory

It is here explained how the Green-Schwarz superstring theory arises from Matrix String Theory. This is obtained as the strong YM-coupling limit of the theory expanded around its BPS instantonic configurations, via the identification of the interacting string diagram with the spectral curve of the relevant configuration. Both the GS action and the perturbative weight $g_s^{-\chi}$, where $\chi$ is the Euler characteristic of the world-sheet surface and $g_s$ the string coupling, are obtained.